Thermal transfer recording is generally carried out by heating a thermal transfer recording material, called an ink ribbon, comprising a support having thereon a color forming layer containing a subliming or vaporizing dye thereby to sublimate or vaporize the dye and transferring the dye to an image-receiving sheet to form a dye image.
More specifically, as shown in FIG. 3 of the accompanying drawings, transfer recording material 1 composed of support 4 and color forming layer 5 and thermal transfer image-receiving sheet 2 composed of thermal transfer image-receiving layer 6 and support 7 are brought into contact between drum 12 and electrically controlled heating source 3, such as a thermal head. Color forming layer 5 of transfer recording material 1 is thus heated by heat source 3 to sublimate or vaporize the dye contained therein, and the sublimated or vaporized dye is transferred to image-receiving layer 6 to accomplish thermal transfer recording.
The material constituting image-receiving layer 6 depends on the kind of the color former (dye) to be transferred thereto. For example, in using a heat-fusible color former, support 7 per se can serve as an image-receiving layer. In using a subliming disperse dye as a color former, a coat layer comprising a high polymer, such as a polyester, may be used as an image-receiving layer.
Support 7 of image-receiving sheet 2 includes pulp paper, opaque synthetic paper comprising a stretched film of a propylene-based resin containing an inorganic fine powder, such as calcined clay or calcium carbonate (as disclosed in U.S. Pat. No. 4,318,950), and a coated synthetic paper prepared by coating a transparent polyethylene terephthalate or polyolefin film with a pigment coating agent containing a binder and an inorganic fine powder, such as silica or calcium carbonate, to impart whiteness and dye-receptivity.
Taking into consideration after-use properties of an image-receiving sheet with a transferred dye image on, for example, suitability to copying, writability with a pencil, and record preservability, a synthetic paper comprising a microvoid-containing stretched film of a polyolefin resin containing an inorganic fine powder is preferred as a support from the standpoint of strength, dimensional stability, and contact with a printing head, as disclosed in JP-A-60-245593, JP-A-61-112693 and JP-A-63-193836 (the term "JP-A" as used herein means an "unexamined published Japanese patent application").
In this type of synthetic paper, microvoids are formed by stretching an inorganic fine powder-containing polyolefin resin film at a temperature lower than the melting point of the polyolefin resin so as to provide opacity, softness to the touch, intimate contact with a printing head, and smoothness in paper feed or discharge.
With the recent rapid advancement in speeding up of printing on a thermal transfer recording apparatus, a thermal transfer image-receiving sheet, particularly one capable of multiple transfer as disclosed in JP-A-63-222891, has been required to provide an image of high density with satisfactory gradation even with a middle tone pulse width (7 to 9 msec).
It is a common knowledge in the art that printing density can be increased with an increase in surface smoothness of an image-receiving sheet. If a compounding ratio of an inorganic fine powder is reduced in an attempt to increase surface smoothness of synthetic paper as a support, the void formed upon stretching will be reduced in number, resulting in a reduction in cushioning effect. It follows that the image density is reduced as observed in Comparative Example 1 of JP-A-63-222891 discussed above.